Phase 2 of #1003: extend the autosave pattern from the Appearance
panel to the Preferences panel so all preference changes are saved
automatically without requiring a manual 'Save Settings' click.
Mirrors the Phase 1 (Appearance) pattern exactly:
- 350ms debounce on field changes (500ms additional debounce on
the bot_name text input — effective ~850ms latency for typing)
- Inline status feedback (saving / saved / failed + retry button)
- Clears dirty flag and hides unsaved-changes bar after successful save
- Password field excluded — still requires explicit save (security)
- Model selector excluded — still requires explicit save
13 fields now autosaving: send_key, language, show_token_usage,
simplified_tool_calling, show_cli_sessions, sync_to_insights,
check_for_updates, sound_enabled, notifications_enabled,
sidebar_density, auto_title_refresh_every, busy_input_mode, bot_name.
i18n keys (settings_autosave_saving/saved/failed/retry) already exist
in all 8 locales from Phase 1.
Co-authored-by: Feco Linhares <feco.linhares@gmail.com>